Kerberos Constrained Delegation
¿Qué es Kerberos Constrained Delegation?
Kerberos Constrained DelegationAn Active Directory delegation mode that allows a service to impersonate users only to a specified set of target services — safer than unconstrained delegation, but still abused via S4U2Self / S4U2Proxy attacks when misconfigured.
Kerberos Constrained Delegation (KCD), introduced with Windows Server 2003, restricts which other services an account can impersonate a user to: the account's `msDS-AllowedToDelegateTo` attribute lists exactly those target SPNs. KCD comes in two flavors. Classic KCD requires Domain Admin to configure the trust on the delegating account. Resource-Based Constrained Delegation (RBCD), introduced in Windows Server 2012, inverts the configuration so the target service controls who can impersonate to it via `msDS-AllowedToActOnBehalfOfOtherIdentity` — useful for cross-forest scenarios. KCD is far safer than unconstrained delegation, but it is not safe by default: attackers with control of an account that has constrained delegation rights can use S4U2Self to mint tickets for arbitrary users (including Domain Admins) and then S4U2Proxy to access the allowed targets, an attack chain documented since the 2018 Elad Shamir 'Wagging the Dog' research. RBCD attacks via control of computer-account writes (CVE-2021-42278/42287, sAMAccountName spoofing) are equally well known. Modern hardening includes Protected Users group membership for sensitive accounts, the 'Account is sensitive and cannot be delegated' flag, and rigorous review of all delegation attributes in AD.
● Ejemplos
- 01
A web application's service account is constrained to delegate only to the SQL Server SPN; the same account cannot impersonate users to other services.
- 02
A red-team that gains write access to a computer object configures it for RBCD and S4U2Selfs into Domain Admin, demonstrating the need to restrict ms-DS-MachineAccountQuota.
● Preguntas frecuentes
¿Qué es Kerberos Constrained Delegation?
An Active Directory delegation mode that allows a service to impersonate users only to a specified set of target services — safer than unconstrained delegation, but still abused via S4U2Self / S4U2Proxy attacks when misconfigured. Pertenece a la categoría de Identidad y acceso en ciberseguridad.
¿Qué significa Kerberos Constrained Delegation?
An Active Directory delegation mode that allows a service to impersonate users only to a specified set of target services — safer than unconstrained delegation, but still abused via S4U2Self / S4U2Proxy attacks when misconfigured.
¿Cómo funciona Kerberos Constrained Delegation?
Kerberos Constrained Delegation (KCD), introduced with Windows Server 2003, restricts which other services an account can impersonate a user to: the account's `msDS-AllowedToDelegateTo` attribute lists exactly those target SPNs. KCD comes in two flavors. Classic KCD requires Domain Admin to configure the trust on the delegating account. Resource-Based Constrained Delegation (RBCD), introduced in Windows Server 2012, inverts the configuration so the target service controls who can impersonate to it via `msDS-AllowedToActOnBehalfOfOtherIdentity` — useful for cross-forest scenarios. KCD is far safer than unconstrained delegation, but it is not safe by default: attackers with control of an account that has constrained delegation rights can use S4U2Self to mint tickets for arbitrary users (including Domain Admins) and then S4U2Proxy to access the allowed targets, an attack chain documented since the 2018 Elad Shamir 'Wagging the Dog' research. RBCD attacks via control of computer-account writes (CVE-2021-42278/42287, sAMAccountName spoofing) are equally well known. Modern hardening includes Protected Users group membership for sensitive accounts, the 'Account is sensitive and cannot be delegated' flag, and rigorous review of all delegation attributes in AD.
¿Cómo defenderse de Kerberos Constrained Delegation?
Las defensas contra Kerberos Constrained Delegation combinan habitualmente controles técnicos y prácticas operativas, como se detalla en la definición.
¿Cuáles son otros nombres para Kerberos Constrained Delegation?
Nombres alternativos comunes: KCD, Resource-Based Constrained Delegation, RBCD.
● Términos relacionados
- identity-access№ 652
Kerberos
Protocolo de autenticación de red basado en tickets que utiliza criptografía simétrica y un Centro de Distribución de Claves de confianza para ofrecer inicio de sesión único seguro.
- identity-access№ 014
Active Directory
Servicio de directorio empresarial de Microsoft para redes Windows que ofrece autenticación, autorización y gestión de políticas centralizadas para usuarios, equipos y recursos.
- identity-access№ 654
Kerberos Unconstrained Delegation
An Active Directory configuration that lets a service receive and store TGTs for any authenticating user, allowing it to impersonate them to any other service — a high-impact misconfiguration repeatedly abused for credential theft and domain compromise.
- attacks№ 651
Kerberoasting
Ataque offline de contrasenas que solicita tickets de servicio Kerberos a cuentas de servicio y rompe la parte cifrada para recuperar la contrasena en claro.
- attacks№ 886
Pass-the-Ticket
Ataque sobre Active Directory que reutiliza un ticket Kerberos robado para suplantar a un usuario o servicio sin conocer la contrasena.
- attacks№ 836
Ataque de NTLM Relay
Ataque adversario en medio (MITRE T1557.001) en el que el atacante reenvia la autenticacion NTLM de una victima a otro servicio para suplantarla sin conocer la contrasena.